This invention is concerned with a method and apparatus for initiating sequential detonation of a series of explosive charges.
It is well known that improved blasting of a rock face can be achieved by arranging explosive charges in rows of spaced boreholes, and by initiating sequential detonation of the charges in each row, and also sequential detonation from one row to another. The purpose of this is to create a xe2x80x9cfree facexe2x80x9d after each explosion before a successive explosion takes place.
In recent years, so-called xe2x80x9celectronic detonatorsxe2x80x9d have been used to an ever-increasing extent, with a view to achieving greater accuracy of control in the time interval between successive detonations. An electrical control circuit is provided to control the initiation of a blasting sequence, and which is intended to trigger detonation of each successive explosive charge at a predetermined time interval after the preceding detonation.
A considerable amount of research work has been carried out into the subject of control of the time interval between successive explosions. First of all, theoretical studies are carried out to determine the most favourable time interval, dependent upon (a) the nature of the rock medium and (b) the spacing-apart of the explosive charges. Then, electronic control apparatus and related software have to be developed, with a view to achieving in practice detonation at successive intervals which correspond as accurately as possible to the theoretically desired time intervals.
The behaviour of an explosively driven vibration in any particular rock medium is complex, and particularly when a sequence of explosively-derived vibrations is applied through the same rock medium, and inter-acting with each other. There is much published literature on the subject, and which might lead one to assume (erroneously) that blasting technology is now an exact science.
It is of course true that use of modern technology can give more efficient fragmentation of rock than the cruder techniques used in the past e.g. by use of fuses, but despite modern technology being available (including design of sophisticated software to control the blasting operation), in practice results can be of variable quality.
A desired fragmentation of a rock medium normally involves production of a major proportion of fragmented rock material reduced in size below a predetermined size, and without generation of (a) substantial amounts of larger fragments and (b) generation of excessive amounts of unusable small fragments and dust.
Furthermore, to the uninitiated, it might be thought that it would be a positive advantage to generate harmonic vibrations in a solid rock medium i.e. so that successive explosively driven vibrations reinforce each other to apply harmonic vibration to the entire rock mass. However, in practice this gives undesirable ground vibrations.
In particular, despite the use of sophisticated blasting techniques i.e. using theoretical calculations plus sophisticated electronic control equipment to implement the theory, it happens from time to time that harmonic vibrations are set-up in a particular rock mass as a result of a controlled sequence of explosions.
The harmonic vibrations may result in undesirable fragmentation of the rock, and also can give rise to significant environmental problems, which may generate unacceptable noise levels being generated and also by potentially damaging ground vibrations. Quarry sites often are located near to buildings e.g. houses or factory buildings, and environmental requirements are that noise and vibration levels must be kept below set limits.
Vibration measurements are normally required, prior to carrying out regular blasting operations, with a view to meeting requirements of local authority or other agencies controlling quarry operations. However, this involves extra costs which many site operators choose not to bear, with consequent adverse effects on residents living or working nearby.
It Is known from U.S. Pat No. 4,725,991 (Shell) that damaging vibrations can be set-up in the ground, during a rock blasting programme, and which can have adverse effect on (a) the quality of the fragmentation, (b) the efficiency of usage of the explosives and (c) the foundations and structure of any nearby buildings. The Shell patent also acknowledges that this subject has been addressed by many learned papers and publications e.g. by the US Departments of Mines, and all are agreed that very complex waveforms (at differing frequencies) are set-up in a rock mass as a result of a series of detonated explosive charges.
Also, it is recognised that relatively low frequency vibrations can have an adverse effect on building walls and foundations (a) from the point of view of horizontal waveform propagation, (b) vertical waveform propagation, and (c) lateral (shaking) displacement of the walls.
There are also desirable time intervals from the point of view of required fragmentation of a rock mass, and efficient usage of explosives.
In the Shell patent reference, while there are many references to the desirability of achieving optimum blasting timing (i.e. for good fragmentation while simultaneously avoiding undesirable ground vibrations affecting buildings), the actual teaching of the Shell patent is:
(a) to carry out a test explosion in a rock mass at a new site;
(b) measure the vibration profiles at selected measuring sites spaced from the test explosion; and,
(c) use mathematical calculations to derive a desired singular best time interval between successive explosions of a series of charges spaced apart in boreholes in the rock mass, derived from best shot vibrational data
The Shell reference teaches an elegant mathematical model utilised to reach the calculation of desired time intervals, but what is an essential aspect of this teaching is that the calculated time interval applies to the entire blasting programme, and which is a constant time interval between successive explosions in the row.
In particular, the teaching of the Shell reference can only deal with one frequency at a time, whereas the invention does not need a xe2x80x9ctest holexe2x80x9d. Further, the Shell reference has the frequency depending upon charge weight.
Therefore, while the present invention is based on a shared recognition of the problem of simultaneously achieving (a) efficient fragmentation and (b) minimising undesirable building foundation-rocking vibrations, the solution offered by the Shell reference is fundamentally different from that provided by the invention.
The present invention thus seeks to alleviate this problem by providing improved and different means to control the timing of a detonation initiation system, with a view to overcoming, or at least mitigating the risk of harmonic vibrations being generated in a rock medium as a result of initiation of a sequential detonation of a series of explosive charges.
According to the invention there is provided a control system for controlling the initiation of detonation of a series of explosive charges spaced apart from each other in boreholes formed in a rock medium to be blasted, said system being operative to apply controlled time difference in the time interval between successive detonations of at least one phase of the series of charges, and which includes at least three successive detonations, so as to reduce the probability of consecutive stimulation and amplification of ground vibration by reason of the detonation of the charges in the rock medium.
Preferably, the system includes an electrically operated control device which is operative to initiate energisation of detonators associated one with each explosive charge in a respective borehole, and time interval control means for controlling the intervals between successive energisation of at least said one phase of the series of charges.
The means whereby the electrically operated control device initiates energisation of successive detonators can take any suitable form, including direct electrical connection lines, radio transmission or through use of xe2x80x9cshock tubingxe2x80x9d systems known per se.
Each detonator may have a respective individual time interval control unit associated with it. Alternatively, a common remote control unit may be provided to apply selected time intervals between successive energisations of the detonators of at least said part of the series of charges.
In a further preferred arrangement, the system includes an electrically operated control device operative to initiate energisation of detonators associated one with each explosive charge in a respective borehole, a sequential generator connected to the control device and which is programmed, or programmable, to cause operation of the control device so that the latter can initiate successive detonations of said one phase of the series of charges; and electronic adjuster means operative to initiate successive energisations of the detonators of at least said one phase of the series of charges at selected time intervals.
The electronic adjuster means may be connected to the control device and be arranged to be operative to apply predetermined adjustments to programmed time intervals set by the sequential generator. Alternatively, the electronic adjuster means may be connected to the sequential generator and be operative to apply predetermined adjustment to programmed time intervals set by the sequential generator.
The selection of the required time intervals will be dependent upon site factors, including (a) the circumstances of the particular rock mass to be blasted, and (b) the distance separating successively detonated explosive charges.
Therefore, in some circumstances, the selection of time intervals will be pre-determined such that successive (different) time intervals of at least one phase of the series of detonations differ from each other, so as to achieve a desired blasting sequence in which the risk of harmonic vibrations being set up in the rock mass is avoided, or at least minimised.
The invention therefore, by electronic means, may deliberately introduce a variable time portion into each successive time interval (in at least one phase) between successive detonations, thereby at least minimising the risk of generation of consecutive stimulation and amplification of harmonic vibrations and thereby inducing vibrational interference through frequency shifting. in the rock medium, while still achieving desired sequencing of explosive charges and fragmentation of the rock medium.
In a system according to the invention, the successive time intervals in at least one part of the series of detonations may be controlled so as to avoid (or at least minimise) the risk of harmonic vibrations (resonance) being set up in the rock mass. The successive grouped time intervals can be the same as each other in some circumstances of a particular rock mass. Alternatively, they may vary from one detonation to another by fixed amounts or by variable amounts, provided that the cumulative effect does not result in generation of harmonic vibrations in the rock mass.
In one preferred arrangement, the successive time intervals may be selected such that successive vibrational waveforms interfere one with another, again with a view to minimise or avoid, the generation of harmonic vibrations in the rock mass. By way of example, for a first time interval of x milliseconds, second and third time intervals could be xc2xdx or xc2xcx respectively.
The first time interval therefore may be set at a minimum period to avoid so called xe2x80x9ccongestionxe2x80x9d in the rock mass, and subsequent time intervals have progressively reduced time periods so as to create interference and thereby reduce the risks of resonant vibrations being set up.
According to a further aspect the invention also provides a method of controlling the initiation of detonation of a series of explosive charges spaced apart from each other in boreholes formed in a rock medium to be blasted, in which there is applied a controlled time difference in the time interval between successive detonations of at least one phase of the series of charges, and which includes at least three detonations, so as to reduce the probability of consecutive stimulation and amplification of ground vibration by reason of the detonation of the charges in the rock medium.